Let's start entering an equation.
Let's try this one. Do you see the small red crosshair?
Sure, over there!
Simply click the mouse to place the crosshair where you want your equation, and begin to type.
Let's make the font a little bigger and bold using the font bar.
Keep in mind, when you enter Mathcad, you are ready to work with math. I'll show you how to enter text later.
When you begin to type, the crosshair turns into a blue vertical bar, which shows your position in the equation.
When you type an operator, such as a "+" sign, a placeholder appears for the next value.
Mathcad automatically creates a fraction when you press the / (divide) key.
If you add any operations right now, they will show up in the denominator. Radicals and exponents also work this way.
To multiply the whole fraction by 3, you must press the spacebar. This moves the insertion point out of the denominator.
Now you can type " * 3", and it will appear correctly.
Now press "=", and there's your answer!
How can I make changes? Say I want to add 9 instead of 2...
Easy! To edit the equation, click to the right of the number you want to change. 
Then press BACKSPACE and type the number you want.
Now click outside of the math region and Mathcad updates the value.